When sheet-like printed materials are separated, there is frequently the problem that the surface of the stack does not form a plane, but is inclined in one or more directions. The size and direction of the inclination can be due to the wariness of the paper. This can also be caused by the presence of inked areas which are irregular in their distribution. In conventional equipment, it is possible for the suction plates to be set parallel by inclining or, respectively, twisting the entire sucker parallel to the surface of the stack. The suckers, set in this manner, are then locked in position after the setting operation. This is carried out so that the setting which has been made is retained during the entire stroke. In this arrangement, the sucker is not able to be altered during the operation. Therefore, it is not possible to adapt the inclination of the suction plate to any changes in the inclination of the stack surface during the operation of the sheet lifting device. In conventional equipment, adjustment is only possible so as to take care of relatively small inclinations of the suction plates. In many cases, there is no possibility of an exact adjustment to the surface of the stack of sheets. Whenever there is a considerable inclination in the stack of sheets, the suction plates may be inappropriate for use during the sheet lifting operations.
Attempts have been made, in the past, to use rubber bands which are slipped over the suction plates so as to achieve an elastic adaptation to the surface of the stack. In this arrangement, there is the disadvantage that the alignment of the suction surface is performed by the action of a force on the surface of the stack. This may mean that two or more sheets are aspirated simultaneously. When this occurs, there is likely to be a stoppage of production.
It is an object of the present invention to provide an apparatus for lifting sheets in which the suction plates are aligned in relation to the stack surface during each lifting stroke without forces acting on the surface of the stack.
This and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.